variations_of_the_earths_magnetic_fieldfandomcom-20200213-history
Historical sources
Historical sources can provide information about the geomagnetic field evolution back to the late Middle Ages. Churches Assuming that church direction was determined with magnetic compasses, geographic church orientation should be connected with the magnetic declination at the construction date. Already Wehner (1905)Wehner, H., 1905. Ueber die Kenntnis der magnetischen Nordweisung im fruehen Mittelalter, IIII. Das Weltall 1820, 319324, 340347, 351356 and Nippold (1916)Nippold, A., 1916. Ein Beitrag zur Frage der Ausrichtung der Kirchenachsen mit dem Magneten. Arch. Naturwissenschaften Tech. 7, 109114, 236244 dealt with this idea, but their hypothesis of cyclic magnetic secular variation can not be proved by current studies. Abrahamsen (1990)Abrahamsen, N., May 1990. Orientation of Romanesque churches in Denmark suggest common use of magnetic compass in the 12th century. Unpublished manuscript determined geographic orientations of 570 Romanesque churches in Denmark from the 12th century. He found a systematic clockwise rotation from geographic East and concluded that about 25 % of the churches were oriented with the aid of a magnetic compass, as magnetic declination varied between 5° and 20° East between 1000 and 1600 AD. We have investigated 124 churches in Lower Austria and 68 churches in northern Germany, all constructed between 1100 and 1900 AD. The statistic evaluation confirms the preferred alignment of naves towards geographic East. The obtained deviations are most likely caused by town- or landscape, since a systematic rotation due to the use of compasses can not be assessed. In only very few cases small deviations from magnetic East for the construction date have been found. Furthermore, the so-called Easting theory is analysed. Easting usually refers to the direction of sunrise at the church patrons day. Even though a few churches indicate patronal orientation, a general trend is not observed for the gathered data. At the moment we are preparing a paper. Maps Printed historical maps occasionally give information about magnetic declination; these can include compass roses with needles pointing to magnetic North or specific values written down. We have conducted online research into historical maps, gathering about 100 maps displaying regions from all over the world from the 16th to 20th century. Then we focused on maps from the Austro-Hungarian Monarchy located in the Austrian National Archive. Up to now about 15 maps from 16th to 19th century have been collected. Navy Magnetic measurements were carried out by the K. K. Navy along the Adriatic coast for navigational as well as scientific purposes. The following surveys are known: From the first period (1806-1823) 54 declination values are given, while Kreil (1850-1854) determined 20 declination, 15 inclination, 18 horizontal and 15 total intensity values (Schellander, 1871Schellander, J., 1871. Magnetische Beobachtungen an den Kuesten des adriatischen Meeres, Jahrbuecher der k. k. Centralanstalt fuer Meteorologie und Erdmagnetismus, Neue Folge, VI. Band, Jahrgang 1869. ). Schellander (1871) provides 19 declination, 17 inclination, 19 horizontal and 17 total intensity values from 1867-1870. In Laschober (1892)Laschober, F., 1892. Magnetische Beobachtungen an den Kuesten der Adria in den Jahren 1889 und 1890, Beilage zu den Mitteilungen aus dem Gebiete des Seewesens, Pola. one can find 38 results for each measurement type, while Kesslitz (1907)Kesslitz, W., 1907. Erdmagnetische Beobachtungen - Bestimmungen der magnetischen Deklination im Oesterreichisch-Ungarischen Kuestengebiete. Beilage zu den Mitteilungen aus dem Gebiete des Seewesens, Pola. performed only 22 declination measurements. Sundials and globes Sundials are reported to be the oldest sources of magnetic declination (Chapman and Bartels, 1962Chapman, S., Bartels, J., 1962, Geomagnetism, Oxford University Press, London). An attached compass indicates the declination for place and year of construction. Georg von Peuerbach's compass from 1451 is the oldest known declination value. At the current time the collection of sundials at the Kremsmünster Monastery is being evaluated. An overview of already investigated sundials can be found in Korte et al. (2009) Korte, M., Mandea, M., Matzka, J., 2009. A historical declination curve for Munich from different data sources, Physics of the Earth and Planetary Interiors 177 (2009) 161 – 172.. Monasteries Kremsmünster Already Korte et al. (2009) have collected several declination values from Kremsmünster. Nevertheless, we are investigating year books, as not all declination values have been gathered. In this case, indications of inclination measurements are also given (Reslhuber, 1854Reslhuber P. A., 1854, Ueber das magnetische Observatorium in Kremsmünster und die vom Jahre 1839-50 aus den Beobachtungen abgeleiteten Resultate, Wien.). Mining In mining activities compasses were often used for orientation purposes. Schreyer (1886)Schreyer O., 1886, Erdmagnetische Beobachtungen im Koenigreich Sachsen. Gerlachsche Buchdruckerei, Freiberg. compiled declination data in Saxony from 1575 to 1885. Christian Doppler (1849)Doppler C., 1849, Ueber eine bisher unbenuetzte Quelle magnetischer Deklinationsbeobachtungen, Sitzungsberichte mth. - nat. Classe, k. Akademie d. Wissenschaften, Wien 2, 249-261 provided also a collection of declination values. However, it has yet to be clarified to what extent available data have already been used by Korte et al. (2009) and Jonkers et. al (2003)Jonkers, A., Jackson, A., Murray, A., 2003. Four centuries of geomagnetic data from historical records. Rev. Geophys. 41, 2, doi:10.1029/2002RG000115.. Observatories and field expeditions Up to now declination values measured by Maximilian Hell in the observatory in Vardoe and on a sea voyage in North Atlantic have been gathered (see Hansen and Aspaas, 2005Hansen T. L., Aspaas P. P., 2005, Maximilian Hell's geomagnetic observations in Norway 1769, Tromso Geophysical Observatoty Reports, No. 2). A detailed anlysis will follow. More sources of these types have not been investigated yet. Established data Historical sources have been already collected by: Jonkers et al., 2003 They have collected more than 170000 magnetic values (around 152000 declinations, 20000 inclinations and 16000 intensities) from 1510 to 1930. The collection includes major published compilations as well as original manuscript sources, mostly from ship's logbooks. Detailed information about data processing as well as the table of data sources can be found in their paper. Jackson et al., 2000Jackson, A., A. R. T. Jonkers, and M. Walker, 2000, Four centuries of geomagnetic secular variation from historical records, Philos. Trans. R. Soc. London, Philos. Trans. Math. Phys. Eng. Sci., 358, 957–990, 2000. This collection was the preliminary work for Jonkers et al. (2003). They present a model of the magnetic field at the core-mantle boundary for the interval 1590-1990. Access to the model gufm1: http://jupiter.ethz.ch/~cfinlay/gufm1.html. Korte et al., 2009 They have compiled 635 historical declination values for southern Germany and surrounding areas investigating sundials, historical maps, monasteries, meteorological stations, mining activities and church orientations. Data sources dating back to the 15th century are presented in their paper. References